The present invention relates generally to imaging inspection systems and methods. Specifically, the present invention relates to an X-ray inspection system using tomosynthesis imaging techniques.
The mounting of Integrated Circuits (xe2x80x9cICxe2x80x9d) chips on Printed Circuit Boards (xe2x80x9cPCBsxe2x80x9d) requires inspection of the interconnections on the PCBs to determine whether the interconnections contain significant defects. Continual increases in the IC chip complexity, performance, and placement density place demands on the density and functionality of package interconnections. The Ball-Grid-Array (xe2x80x9cBGAxe2x80x9d) is one example of a Surface-Mount-Technology (xe2x80x9cSMTxe2x80x9d) package with interconnections that demand specialized inspection techniques. The continually increasing complexity and density of the PCB interconnections have resulted in the development of a number of interconnection inspection techniques for detecting defects on or within the interconnections.
One such interconnection inspection technique, tomosynthesis, is capable of detecting defects by creating a digital image representation of a sliced view along a single plane passing through a three-dimensional electrical solder joint connection. A digital tomosynthesis system makes it possible to inspect various PCB solder joint qualities, which cannot be inspected by visual methods or conventional X-ray radiography methods. U.S. Pat. No. 4,688,241 issued on Aug. 18, 1987 to Richard S. Peugeot, incorporated herein by reference, discloses a number of tomosynthesis inspection systems, including a system 10 depicted in FIG. 1 of the instant application. The system 10 includes a steerable microfocus X-ray source 12, a large-format image detector 30 capable of imaging X-rays, and an inspection plane 20 positioned between the source and the detector. As used herein, the term xe2x80x9csteerablexe2x80x9d in reference to the source 12 refers to the capability to direct an electron beam within the source 12 to various locations on a target anode. In contrast, a stationary or non-steerable source, as used herein, refers to a source that lacks such capability, i.e. the electron beam strikes the target anode at a single location.
The regions A, B, and C to be imaged may be placed on an X-Y table (not shown), which lies in the inspection plane 20. When an object is on the X-Y table, the test object may be translationally moved along the x and y directions so that a region of interest, such as a solder joint, can be imaged. The source 12 produces an X-ray beam 50 having sufficient energy to penetrate the test object and reach the detector 30, while also having a low enough energy so that a resulting image has contrast within the region of interest.
The X-ray source 12 and the detector 30 may be mounted on independent vertical drive mechanisms allowing a continuously variable field-of-view, ranging from approximately 2.5 mmxc3x972.5 mm to approximately 25 mmxc3x9725 mm, to be obtained. In particular, the X-ray source 12 is mounted on a programmable Z-axis, which changes the distance between the X-ray source 12 and the inspection plane 20. The distance between the X-ray source 12 and the plane 20 is referred to herein as Z1. The detector is also mounted on a programmable Z-axis, which changes the distance between the inspection plane 20 and the detector 30. The distance between the inspection plane 20 and the detector 30 is referred to herein as Z2. Variation of the field of view may be accomplished by varying either or both distances Z1 and Z2.
The operation of the system of FIG. 1 now will be explained. A circuit board having regions of interest A, B, and C is positioned on the X-Y table, in the inspection plane 20. The board is then moved translationally along the x and y directions so that a region of interest A, B, or C, such as a solder joint, or a component can be imaged. Once the board is properly positioned, a beam of radiation, such as X-ray beam 50, is projected towards an object on the circuit board. A portion of the X-ray beam 50 transmits through and is modulated by the object.
The portion of the beam 50 that passes through the object then strikes the image detector 30. The detector 30 is capable of producing an X-ray shadowgraph containing the modulation information from the test object. The X-rays striking the input screen of the detector 30 produce a visible light or shadowgraph image of the volume of the object that falls within the X-ray beam 50. If the detector 30 includes an image intensifier, the image at the output of the image intensifier is amplified in brightness.
The image that appears on the output face of the detector 30 is viewed, through a mirror, by a video camera (not shown). The images from various regions of the detector 30, such as the regions numbered 1, 3, 5 and 7 in FIG. 1, may be sequentially directed to the camera by adjusting the position of the mirror.
The resulting images are then input into a video digitizer. The video digitizer provides as an output digitized image sets. Each image in the set is supplied to a memory and stored. The images may then be separately fed into a tomosynthesis computer, which is programmed with a known tomosynthesis algorithm that effects a combination of the images and provides a resultant image to a monitor. In order to improve the resolution of the digitized image sets, it is desirable to limit the field of view of the camera to a region of the detector 30, such as the regions 1, 3, 5 or 7, rather than to acquire images for tomosynthesis viewing the entire detector 30.
For system 10, the center of the region of interest must coincide with a line extending from the center of the path of the x-ray source to the center of the detector 30. As can be seen in FIG. 1, the center of object B coincides with the centerline of X-ray beam 50 and the center of the field of view of detector 30.
To acquire tomosynthetic images for object B, for example, the X-ray source 12 is positioned at multiple points 1-8 along a circular path that is perpendicular to the Z axis. Each point on the circle falls in a plane that is perpendicular to the Z axis and maintains the same angle with, or is equidistant from, the Z axis. At each point, the X-ray source 12 emits an X-ray beam 50 towards, and at least partially through, the object B, thereby generating an image of object B at the detector 30. For example, to acquire image 1 for object B, the X-ray source 12 is steered to position 1 and the detector field of view is moved to position 1. This process is repeated for images 2 through 8 of object B. The 8 images are acquired sequentially since the electron beam inside the X-ray source housing and the detector field of view must be moved after each acquisition. As a result, 8 scanned images of object B at a known pre-determined angle are captured.
After the required images of object B are taken, then the X-Y table is moved so that the center of object A coincides with the centerline of the X-ray beam 50 and the center of the detector field of view. To acquire image 1 for object A, the X-ray source 12 is steered to position 1 and the detector field of view is moved to position 1. This process is repeated for images 2 through 8 of object A. Thus, 8 scanned images of object A are captured. This process is continued for each of the objects, or regions of interest, to be imaged.
In order for tomosynthesis to be effective, the angle phi should be at least a 25-30 degree angle from perpendicular to generate a useful tomosynthetic slice of the object. However, the practical limitations of the diameter of the X-ray source, the diameter of the detector, the distance between the source and the object, Z1, and the distance between the object and the detector, Z2, result in compromises to be made with respect to the angle that can be achieved, the field-of-view, the resolution, and the speed of the system. In order to achieve the desired angle and thus a useful tomosynthetic slice, a costly X-ray source and/or detector are required.
As mentioned above, conventional tomosynthesis techniques, such as those shown by Peugeot in U.S. Pat. No. 4,688,241 and depicted in FIG. 1, require that the centerline of the X-ray focal spot position and of the field of view at the detector is coincident with the center of the object to be imaged. There are a number of resultant advantages from this arrangement. Passing the X-ray beam through the center of the region of interest simplifies calibration of the machine, the dewarping and gray correction of the images, and the mechanical positioning of the object. The quality of the tomosynthetic slice depends on accurate positioning of the electron beam and mirrors. This accuracy can be achieved with existing technology for electromagnetic beam steering and galvonometer mirrors. A disadvantage of conventional systems, however, is that they require the use of a large-format detector and a steerable X-ray source. Such equipment is expensive and its use increases the overall cost of the system. Further, with such systems, it is slower to sequentially acquire each one of the 8 images, thus limiting the speed of the system to 8 times the time it takes to acquire one image.
Thus, there is a need for an X-ray inspection system using a tomosynthesis imaging technique that does not require the centerline of the X-ray focal spot position and of the field of view at the detector to be coincident with the center of the object to be imaged.
There is a further need in the art for an X-ray inspection system using a tomosynthetic imaging technique that does not require both a steerable X-ray source and a large-format detector.
There is yet a further need in the art for an X-ray inspection system using a tomosynthesis imaging technique that increases the throughput of the system while decreasing its overall cost.
The present invention meets the needs of the prior art by providing an X-ray inspection system using a tomosynthesis imaging technique that does not require the centerline of the X-ray focal spot position and the field of view at the detector to be coincident with the center of the object to be imaged. With this requirement eliminated, significant cost and performance advantages are realized by avoiding the use of either a large-format detector or a steerable X-ray source or both.
These advantages can be achieved by using an inspection system having a non-steerable X-ray source combined with an X-ray detector that can capture 8 images simultaneously. Thus, the need for a costly steerable X-ray source is eliminated and the overall system is simplified. Further, the speed or throughput of the system is improved.
These advantages can be alternatively realized by using an inspection system having a steerable X-ray source and a small-format high-resolution detector. By steering the X-ray source further off-center, the image of the object can be projected onto a high resolution, small-format detector. Thus, cost savings are achieved by using a smaller, less expensive detector.
Since X-ray detectors and steerable X-ray sources are typically the most expensive components in the inspection system, decreasing the cost of one or both would decrease overall system cost while still maintaining the required performance.
The present invention also reduces the number of mechanical repositioning movements required to place the X-ray source or the detector and the target object in position for tomography. Therefore, the present invention enables images of complex interconnections to be created in less time with less expense.